3D multiscale analysis of the hierarchical porosity in Coscinodiscus sp. diatoms using a combination of tomographic techniques.

A full 3D analysis of the hierarchical porosity in Coscinodiscus sp. diatom structures was carried out by using a multiscale approach that combines three advanced volumetric imaging techniques with resolutions and fields of view covering all the porous characteristics of such complex architectures: electron tomography, "slice and view" approach that uses a dual-beam microscope (FIB-SEM), and array tomography consisting of serial imaging of ultrathin specimen sections. This multiscale approach allowed the whole porosity network to be quantified and provided an unprecedented structural insight into these natural nanostructured materials with internal organization ranging from micrometer to nanometer. The analysed species is made of several nested layers with different pore sizes, shapes and connectivities and characterized by the presence of interconnected pores structured in various ways. The first evidence of the presence of a nanometric porosity made of ellipsoidal pores in the siliceous diatom frustules is also provided.

Local Porosity Measurement From Scanning Electron Microscopy Images in the Backscattered Electrons Mode.

We propose a method to measure the local porosity of porous samples from scanning electron microscopy images in the backscattered electron mode. The porous samples are impregnated with a polymer resin and observed in polished cross sections. Image intensities are calibrated with intensities from pure resin and the bulk phase. The calibration model is justified with Monte Carlo simulations on perfectly homogeneous virtual samples. Uncertainties in measured porosity are given as a function of uncertainties on physical properties of the resin and the bulk phase and on measured signals. The methodology is applied to a series of heterogeneous alumina catalyst supports with varying porosities. A good agreement is found between the averaged local porosity by scanning electron microscopy and global porosity determined by mercury intrusion porosimetry. The use of local porosity statistics allowed the quantitative characterization of the porosity fluctuations of these supports that appeared to be linked with their preparation parameters.